Two Sum with a Dictionary

Console.WriteLine(string.Join(", ", TwoSum(new[] { 2, 7, 11, 15 }, 9)));    // 0, 1
Console.WriteLine(TwoSum(new[] { 1, 2, 3 }, 10).Length);                    // 0

// One pass. For each value, check whether the number that completes the sum was seen earlier.
// O(n) time and O(n) space, instead of O(n squared) with two nested loops.
static int[] TwoSum(int[] numbers, int target)
{
    var indexByValue = new Dictionary<int, int>();
    for (int i = 0; i < numbers.Length; i++)
    {
        int complement = target - numbers[i];
        if (indexByValue.TryGetValue(complement, out int j)) return new[] { j, i };
        indexByValue[numbers[i]] = i;
    }
    return Array.Empty<int>();
}